Hikmet's repitition of "Living is no laughing matter" creates an accentuation of his message to life life with a purpose and without regrets.
Hikmet exposes the blindness of mankind relating to death, their denial of the inevitable, their "fear [of] death [even though they] don't believe it". He believes that mankind lives life as it is forever, they don't take advantage of what limited time they have as they cannot wrap their head around that time ending.
Hikmet's saddening scenario, in which "we might not get up", he creates realization within the reader, that our time is limited and to take advantage of what life we do have left.
By relating back to his time in prison and how that time created his own sense of urgency regarding life, Hikmet can pass on those learnings to his audience, urging them to make use of their freedom "beyond the walls" and time.
Hikmet uses imagery to bring an urgency to his message an by saying that the "earth will grow cold", he shows how little time we have and the inevitability of its end.
By italicizing "this" , Hikmet creates an effect of the narrator telling the reader to look around and embrace "our great Earth".